1. Field of the Invention
The present invention relates generally to a communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving an uplink spreading signal in a communication system.
2. Description of the Related Art
Current communication systems transmit/receive signals using, for example, Orthogonal Frequency Division Multiplexing (OFDM) and/or Orthogonal Frequency Division Multiple Access (OFDMA). In addition, the communication systems use, for example, Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat Request (HARQ), multi-antenna, etc. taking into account a high-speed service supporting capability, a bandwidth utilization efficiency, etc.
The communication system includes a transmitter and a receiver, for example, a Mobile Station (MS) and a Base Station (BS), for transmitting/receiving signals. A description will now be given of a signal transmission from the MS to the BS. A signal transmitted from the MS includes control information being fed back to the BS. The control information is information which is fed back from the MS to the BS due to the application of AMC, HARQ, multi-antenna, etc. and includes, for example, Channel Quality Information (CQI), Acknowledgement/Non-Acknowledgement (ACK/NACK), Multiple Input Multiple Output (MIMO) mode switching information, etc.
The MS spreads the control information with a spreading code to generate a spreading signal (or spread signal), and transmits the generated spreading signal to the BS. Therefore, MSs of the communication system can transmit control information to the BS using the spreading code.
To this end, the communication system reserves uplink signal transmission resources for the transmission of the spreading signal. The MSs transmit their generated spreading signals to the BS over the reserved resource region. A description will now be made of a spreading signal transmission region reserved for the transmission of control information, i.e., spreading signal, in uplink resources of the communication system.
FIG. 1 illustrates an uplink frame including a spreading signal transmission region in a conventional communication system.
Referring to FIG. 1, an uplink (UL) frame 111 includes a spreading signal transmission region 113 for transmission of control information that MSs each feed back to a BS. Herein, the control information is a spreading signal which is spread with a spreading code.
The uplink frame 111, as illustrated in the drawing, includes a plurality of symbols and a plurality of subcarriers. Therefore, the MS can transmit a control signal such as the Channel Quality Information (CQI), Acknowledgement/Non-Acknowledgement (ACK/NACK), and Multiple Input Multiple Output (MIMO) mode switching information over the spreading signal transmission region 113.
In the communication system, an index of an MS is assumed herein to be ‘i’. A spreading signal corresponding to the feedback information that an MS #i transmits to the BS is defined herein as Ci,n, where n denotes a value between 0 and N−1. For generation of the spreading signal, the MS performs Binary Phase Shift Keying (BPSK) modulation on a spreading code with an N-subcarrier length.
The spreading signal transmission region 113 includes N subcarriers. The parameter N is a multiple of the minimum uplink resource allocation unit Nalloc, and Nalloc, has various values greater than ‘1’ for each communication system. If the N subcarriers are defined as Sn, n has a value between 1 and N−1. The N subcarriers can be adjacent to each other or distributed over the full band according to the resource structure of the uplink frame.
The signals Sn that MSs transmit over the spreading signal transmission region 113 can be expressed as Sn=Ci,n.
Assume that the number of MSs transmitting uplink feedback information is I and that the MSs each transmit one spreading signal. In this case, if uplink power control is performed in the ideal channel environment, signals received at the BS are expressed as Equation (1).
                              S          n                =                              ∑                          i              =              0                                      I              -              1                                ⁢                                          ⁢                      C                          i              ,              n                                                          (        1        )            
The maximum number of spreading signals transmittable through the signal Sn should be limited such that the BS receiving the spreading signals can detect each spreading code at a specific error rate through despreading. Since the despreading process and an operation of finding a detection probability depart from the scope of the present invention, a detailed description thereof will be omitted herein for simplicity.
However, it is possible to set the maximum number Imax of spreading signals separately for each communication system. Therefore, in the case where the communication system satisfies Sn=Ci,n (where n=0˜N−1), if the number of spreading signals corresponding to the uplink control information is assumed to be I, the amount of subcarrier resources to be allocated in the spreading signal transmission region 113 is expressed as Equation (2).┌I/Imax┐×N  (2)
In Equation (2), ┌x┐ denotes the minimum integer greater than x.
With reference to FIG. 2, a description will now be given of a spreading signal transmission region that results from increasing the amount of resources.
FIG. 2 illustrates a spreading signal transmission region for the transmission of control information in a conventional communication system.
Referring to FIG. 2, MSs use a first spreading signal transmission region 210 and a second spreading signal transmission region 250, for the transmission of control information. A BS receives the spreading signal from MSs over the first spreading signal transmission region 210 and the second spreading signal transmission region 250.
When using the first spreading signal transmission region 210, MSs each transmit a maximum of Imax spreading signals. When the number of spreading signals included in one spreading signal transmission region exceeds a specific threshold, it is impossible for the BS to receive the spreading signal(s) that exceeds the threshold in number. Therefore, the number of spreading signals that MSs can transmit over one spreading signal transmission region is previously determined.
Assume that a signal Sn is transmitted over the first spreading signal transmission region 210 and that the signal Sn includes N subcarriers. Imax signals 211, 213 and 215 transmitted over the first spreading signal transmission region 210 are shown.
Assume that the number of spreading signals that MSs will transmit to the BS is Imax+1. In this case, because it has exceeded the maximum number of spreading signals transmittable over one spreading signal transmission region in an uplink frame, the BS should additionally use another spreading signal transmission region, i.e., the second spreading signal transmission region 250, for transmission of the exceeding spreading signal, i.e., one spreading signal 251.
Like this, even though the number of spreading signals that exceeds the maximum number Imax of spreading signals is only one, there is a need for a separate second spreading signal transmission region 250 which is equal in size to the first spreading signal transmission region 210. As a result, in order to transmit the Imax+1 spreading signals, there is a need for 2N subcarriers, assuming that the number of resources, i.e., subcarriers, of each spreading signal transmission region is N.
Therefore, for the transmission of the spreading signals, where there is a number of spreading signals which exceed the maximum number of spreading signals to be transmitted over one spreading signal transmission region with a preset size, it is necessary to additionally use a separate spreading signal transmission region. In other words, there is a need to use a spreading signal transmission region with a preset size regardless of the number of the exceeding spreading signals, causing a waste of resources.